| Crofton weed, Ageratina adenophora (Sprengel) (Synonym: Eupatorium adenophora) (Asteraceae), is a noxious invasive alien species, and it has become widespread in many areas in China including Yunnan, Sichuan, Guangxi, Guizhou and Chongqing, and caused seriously economic losses and ecological threat. The successful invasion of the weed might be partly due to its strong allelopathy. The aqueous leachates and root exudates from crofton weed were conducted according to the releasing routes of plant allelochemicals in nature conditions. And the allelopathic effects of the aqueous extracts and root exudates on several receptor plants were studied using the bioassay method in lab. Subsequently, potential allelochemicals of aqueous extracts and root exudates were analyzed by GC-MS. The main results showed as follows.1.The allelopathic effects of the aqueous leachates from A. adenophora on six receptor plants, cucumber (Cucumis sativas), clover (Trifolium repeus), purple medic (Medicago sativa), lettuce (Lactuca sativa), ryegrass (Lolium perenne), barnyardgrass (Echinoloa crusgalli), were bioassayed in lab, the results indicated that germination and seedling growth were significantly inhibited, the inhibiting effects of aqueous extracts to receptor seedling were stronger than germination. The influence to the seed had two forms, inhibiting and delaying germination. The length of shoots and roots was inhibited and the seedling had malformed symptom. Allelopathy of aqueous leachates of the upper-ground part was stronger than the under-ground part in the same concentration. The allelopathic effect on the shoots of receptor plants was stronger than that of roots. This allelopathic effect was influenced by the concentration of the aqueous leachates, and changed with different receptor plants. The length of radical elongation and shoot, fresh weigh of lettuce increased with the treatments of the aqueous extracts at the concentrations of 0.005 gDW/ml and 0.0005 gDW/ml, whereas, decreased with higher concentrations (0.05 g DW/ml) . The growth of purple medic also was inhibited by aqueous extracts at the concentrations of 0.05 g DW/ml. The growth of cucumber, clover, ryegrass and barnyard grass was inhibited at the concentration of 0.05 g DW/ml, and was concentration dependent. It was considered that the indices of germinating speed was more suitable to evaluate potential allelopathy than the rate of seed.2. Allelopathic effect of the aqueous leachates on plants of ryegrass and purple medic were bioassayed in lab, the results showed the length of radical elongation and shoot, fresh weigh, and the total and the active uptake area of roots of receptor plants were inhibited when the treatment concentration was higher than 0.01 gDW/ml to the ryegrass and 0.025 gDW/ml to the purple medic.3. The allelopathic effects of decomposition substance of fallen aerial parts from crofton weed were studied in lab, the results showed that the rate of germination, index of germinating speed, length of root and shoot and fresh weight of receptor plants (cucumber, Chinese cabbage, clover and ryegrass) were inhibited significantly when the treatment concentration at 0.01-0.05 gDW/ml.4. The fraction at the concentration of 10 gFW/ml was extracted with dichloromethane from root exudates, showed significant inhibition action to seed germming and seedling growth of purple medic, ryegrass and clover.5. The acidic, neutral, alkalescence and XAD-4 resin fractions extracted by ethyl acetate from the aqueous extracts, showed significant inhibition action on seed germination and seedling growth, of purple medic, ryegrass and cucumber. Compared with the other fractions, the neutral fraction showed the biggest inhibition action.6. The four fractions were analyzed by GC-MS. The results indicated that the neutral fraction contained eleven components. The eleven components were ester and terpene mainly, in which five components are the 9-oxo-ageraphorone and its ramification. The, alkalescence, acidic and XAD-4 resin fraction contained 5, 4 and 3 components;those were contained in the neutral fraction. By bioassay, the 9-oxo-ageraphorone and its ramification are the main allelochemical of crofton weed. Main chemical compounds in the extraction of dichloromethane from root exudates by GC-MS analysis are ester substance.The dried-up plants of crofton weed and the root exudates contain some allelochemical, which can inhibit seed germination and seedling growth of receptor plants. The routs of the stem and leaf elucidation are the main form of allelopathy. The decomposition substance of fallen aerial parts from A. adenophora is other resource of allelochemicals, and also the same from the root exudates. Our results showed the action of allelopathy play an important role in the invasion of A. adenophora, which enriched the "New weapon hypothesis" of invasive alien weed and offered theoretic basis to the invasive mechanism of alien weed.
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